Xinyuan Miao1,2, Yuankui Wu1,2,3, Dapeng Liu1,2, Hangyi Jiang1, Qin Qin1,2, Peter C.M van Zijl1,2, Jay J. Pillai4,5, and Jun Hua1,2
1Neurosection, Division of MRI Research, Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, Baltimore, MD, United States, 2F.M. Kirby Research Center for Functional Brain Imaging, Kennedy Krieger Institute, Baltimore, MD, United States, 3Department of Medical Imaging, Nanfang Hospital, Southern Medical University, Guangzhou, China, 4Johns Hopkins University School of Medicine, Division of Neuroradiology, Russell H. Morgan Department of Radiology and Radiological Science, Baltimore, MD, United States, 5Department of Neurosurgery, Johns Hopkins University School of Medicine, Baltimore, MD, United States
Synopsis
Metallic
objects such as dental braces bring substantial susceptibility artifacts in MR
images acquired using echo-planar-imaging (EPI) sequences. Here, we demonstrate
that diffusion-prepared diffusion tensor imaging (DTI) with three-dimensional
fast gradient-echo readout can significantly reduce susceptibility artifacts
that are commonly seen in conventional spin-echo (SE) EPI DTI in the presence
of metallic orthodontic braces.
Introduction
Dental fillings and orthodontic braces containing various metals
can cause large susceptibility artifacts extending from the facial region into
the brain1,2,3 in commonly used
echo-planar imaging (EPI) based sequences. This is particularly a problem for
diffusion-tensor-imaging (DTI) MRI studies4, where spin echo (SE)-EPI sequences are commonly used. Such
susceptibility artifacts include signal dropout and geometric distortion in
affected regions. SE-EPI in principle has less signal dropouts than gradient
echo (GRE) EPI. However, geometric distortion due to local B0 inhomogeneity can
still be significant in SE-EPI. Therefore, many DTI studies choose to acquire
two identical DTI scans with opposite phase encoding directions in order to
correct for such distortion, at the expense of doubling scan time and
introducing potential motion and physiological variations between scans5. Alternatively, diffusion contrasts can be induced using spin
preparation modules before readout6,7 which separates contrast generation from the readout, thereby
opening the possibility to use virtually any pulse sequence for image
acquisition. Among the various strategies, a double refocusing diffusion
preparation module followed by a 3D readout has been commonly adopted8,9, which can significantly reduce image distortion commonly seen in
EPI10. In this study, we applied a diffusion-prepared-DTI approach8,11 with single-shot 3D fast-gradient-echo (GRE) readout in healthy
human subjects wearing metallic orthodontic braces to evaluate its ability to
minimize susceptibility artifacts in the presence of metallic objects12.Methods
Six healthy participants
(40±6yo, 3 females) were recruited for this study. Removable dental braces with
bonding trays were used so that MRI images can be acquired with braces or
without braces in the same participants. Figure
1 illustrates the DTI sequences. The diffusion-prepared DTI sequence
includes a diffusion preparation module for generating the desired diffusion
contrast, followed by a single-shot 3D fast GRE readout. The diffusion
preparation module uses double refocusing
pulses, with additional diffusion weighting gradients inserted between the RF
pulses6,7,13,14. To minimize eddy current related artifacts and reduce T1 effects
during the readout, a stimulated echo scheme8 is adopted: a dephasing gradient in the slice-encoding direction before
the last 90° pulse in the diffusion preparation module, and a set of rephasing
gradients in the readout (each of which has the same area as the dephasing
gradient) are added. The following scans were acquired for each subject on a 3T
Philips MRI scanner: MPRAGE (voxel=1x1x1mm3); SE-EPI DTI: b=0 and
800s/mm2, 15 diffusion gradient directions voxel=2.5x2.5x2.5mm3;
diffusion-prepared DTI: same b-values, diffusion gradients and voxel size as
SE-EPI DTI, TR/TE(effective)=5000/90ms, FA=11°, SENSE factor=3x1, centric order,
TRGRE/TEGRE=4.1/2.1ms, dephasing gradient=2mT/m and 5ms. DTI
data were processed using MRI-Studio (www.mristudio.org). Signal-to-noise-ratio
(SNR), apparent-diffusion-coefficient (ADC) and fractional-anisotropy (FA) were
compared in two manually drawn ROIs: bilateral inferior fronto-occipital
fasciculus (IFOF) with strong susceptibility artifacts, and bilateral posterior
limb of internal capsule (PLIC) with minimal susceptibility artifacts in
EPI. Geometric distortion was visualized
using Slicer in FSL (https://fsl.fmrib.ox.ac.uk/), and quantified by the
Jaccard index, which ranges from zero to one, indicating no overlap to complete
agreement, respectively, between the geometric shapes of the DTI image and
reference structural images.Results
Figure 2 shows typical raw
diffusion-weighted images, ADC, and color-coded FA maps from one subject
wearing braces. While SE-EPI shows signal loss in many brain regions (e.g. the
frontal lobe in the slice shown), no obvious artifacts were visible in the diffusion-prepared
EPI in the entire brain. Color coded FA maps obtained from SE-EPI
DTI showed spurious results in the inferior frontal lobe near the brace (red
arrow in Figure 2), affecting
visualization of the IFOF, as compared to diffusion-prepared color FA maps in
the same subject. Geometric distortion (Figure
3) was minimal in diffusion-prepared DTI, but was substantial in SE-EPI DTI
(significantly lower Jaccard index in each slice, P < .001), and the degree
of distortion varied with the location of the slice. Table 1 summarizes the group-averaged quantitative results from all
subjects from the ROI analysis (ROIs delineated in Figure 2). ADC, FA and SNR values were all comparable between
diffusion-prepared and SE-EPI in the PLIC, a structure minimally affected by
the susceptibility artifacts. In the IFOF, which is close to the dental braces,
SNR was significantly diminished in SE-EPI, leading to erroneous ADC and FA
values, whereas diffusion-prepared DTI showed greater SNR and reasonable ADC
and FA values consistent with the literature16. When the
same scans were repeated in the same subjects without wearing the metallic
dental braces, ADC, FA and SNR in both the PLIC and the IFOF became comparable
between SE-EPI and diffusion-prepared DTI scans, all of which are within the
typical range reported in the literature.Discussion & Conclusion
We demonstrate that
diffusion-prepared-DTI can acquire diffusion MR images in healthy human
subjects wearing metallic dental braces with preserved SNR for the entire
brain, whereas conventional SE-EPI DTI showed significantly reduced SNR in
regions with strong susceptibility effects. Also, the geometric distortion was
reduced significantly with diffusion-prepared DTI. This technique is expected
to provide an alternative approach in studies involving regions with large susceptibility artifacts caused by metallic implants
in the brain.Acknowledgements
NINDS (1R01NS108452),
NIBIB (R21EB 023538 and P41 EB015909), NICHD (U54 HD079123).References
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